Polymers which are designed for base-specific binding to polynucleotides have significant potential both for in vitro detection of specific genetic sequences characteristic of pathogens (Lerman) and for in vivo inactivation of gene tic sequences causing many diseases--particularly viral diseases (Belikova, Summerton).
Standard ribo- and deoxyribonucleotide polymers have been widely used both for detection of complementary genetic sequences, and more recently, for inactivating targeted genetic sequences. However, standard polynucleotides suffer from a number of limitations when used for base-specific binding to target oligonucleotides. These limitations include (i) restricted passage across biological membranes, (ii) nuclease sensitivity, (iii) target binding which is sensitive to ionic concentration, and (iv) susceptibility to cellular strand-separating mechanisms.
In principle, the above limitations can be overcome or minimized by designing polynucleic acid analogs in which the bases are linked along an uncharged backbone. Examples of uncharged nucleic acid analogs have been reported. Pitha et al (1970a, b) have disclosed a variety of homopolymeric polynucleotide analogs in which the normal sugar-phosphate backbone of nucleic acids is replaced by a polyvinyl backbone. These nucleic acid analogs were reported to have the expected Watson/Crick pairing specificities with complementary polynucleotides, but with substantially reduced Tm values (Pitha, 1970a). One serious limitation of this approach is the inability to construct polymers by sequential subunit addition, for producing polymers with a desired base sequence. Thus the polymers cannot be used for base-specific binding to selected target sequences. Polynucleotide analogs containing uncharged, but stereoisomeric, methylphosphonate linkages between the deoxyribonucleoside subunits have been reported (Miller, 1979, 1980; Jayaraman; Murakami; Blake, 1985a, 1985b; Smith). More recently a variety of analogous uncharged phosphoramidate-linked oligonucleotide analogs have also been reported (Froehler, 1988). These polymers comprise deoxynucleosides linked by the 3'OH group of one subunit and the 5' OH group of another subunit via an uncharged chiral phosphorous-containing group. These compounds have been shown to bind to and selectively block single-strand polynucleotide target sequences. However, uncharged phosphorous-linked polynucleotide analogs of the type just described have limitations, particularly the cost and difficulty of preparing the polymers.
More recently, deoxyribonucleotide analogs having uncharged and achiral intersubunit linkages have been constructed (Stirchak 1987). Since these polymers are stereo-regular, all polymers having a given subunit sequence will have the same Tm value for a given target nucleotide sequence, thus avoiding some of the limitations inherent in chirally-linked polymers. These uncharged, achiral deoxyribonucleoside-derived analogs, however, are limited by relatively high cost of starting materials.